class Solution {
    int dx[4]={0,0,1,-1};
    int dy[4]={1,-1,0,0};
public:
    int maxDistance(vector<vector<int>>& grid) {
        int m=grid.size(),n=grid[0].size();
        vector<vector<int>> dist(m,vector<int>(n,-1));
        queue<pair<int,int>> q;

        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++)
                if(grid[i][j])
                {
                    dist[i][j]=0;
                    q.push({i,j});
                }
        int ret=-1;
        while(q.size())
        {
            auto [a,b]=q.front();q.pop();
            for(int i=0;i<4;i++)
            {
                int x=a+dx[i],y=b+dy[i];
                if(x>=0 && x<m && y>=0 && y<n && dist[x][y]==-1)
                {
                    dist[x][y]=dist[a][b]+1;
                    q.push({x,y});
                    ret=max(ret,dist[x][y]);
                }
            }
        }
        return ret;
    }
};
